1. Field of the Invention
The present invention relates to a a digital adaptive filter and method of convergence therein. More particularly, it relates to an apparatus and a method for cancelling an echo component generated by a partial return of a transmission signal to a receiving side by a digital adaptive filter.
When a bidirectional data transmission is effeted through a hybrid circuit, the transmission signal is partially returned to the receiving side through the hybrid circuit to thereby cause a reception error, and therefore, the echo component generated by the partial return of the transmission signal to the receiving side must be cancelled.
2. Description of the Related Arts
A digital adaptive filter is used to cancel the echo component, and conventional digital adaptive filters are classified into two types, i.e., a look-up table type echo canceller and a transversal filter type echo canceller.
In the look-up table type echo canceller, a sequence of transmission signals is guided to a tap portion, and a pattern of the sequence of signals is used as an address when storing in a memory portion a sequence of pseudo echoes having an opposite polarity to an echo value. Accordingly, if an echo of the same address subsequently input, the echo is cancelled by the pseudo echo stored in the memory.
The transversal filter type echo canceller employs a known transversal filter to converge the input signal. One tap coefficient is provided N pieces of input data, and according to the pattern of the tap coefficients and transmission symbols, pseudo echoes are generated to cancel echoes.
The look-up table type echo canceller is advantageous for cancelling non-linear echo components, but the memory capacity thereof must be increased as the tap length of an echo becomes longer, thus causing a disadvantage in that an initial convergence time (a training time) may become longer. Particularly this disadvantage will be serious when multivalued codes are employed as transmission codes.
The transversal filter type echo canceller is advantageous in that echoes having long tap lengths are cancelled within a short convergence time, but has a disadvantage in that non-linear echo components cannot be cancelled thereby.
Therefore, the above digital filters are rarely employed to cancel echoes containing non-linear components and having relatively long tap lengths.
To cope with this problem, a memory division type digital filter having a look-up table memory divided into a main memory portion and a plurality of submemory portions was provided prior to the present invention.
The memory division type digital filter ebables a reduction of the memory capacity, as described in more detail later with reference to the drawings, but this arrangement incurs a division loss, i.e., an accumulated error appearing as a residual echo component.
It may be considered to combine the above memory division type filter with the transversal filter type echo canceller, but in this combination a suitable converging algorithm that can accurately carry out the initial convergence and adaptive control is not known.